Gluconeogenesis 1: Study with Video Lessons, Practice Problems & Examples

Practice - Gluconeogenesis

Gluconeogenesis is often thought of as simply the reverse of glycolysis, and this is mostly true. As you probably realize, many of the reactions are readily reversible. So, many of the same enzymes used in glycolysis are actually used in gluconeogenesis. They're simply doing the reverse reaction that they do in glycolysis, meaning that again, they're readily reversible. However, remember, there are those 3 reactions from glycolysis, reactions 1, 3, and 10, that are too favorable. They have too negative ΔG. And so new enzymes are going to be required to undo the actions of those particular reactions. And we'll get to that on the next page. For now, let's take more of a top-down look at gluconeogenesis. So just like glycolysis, it has many different molecules that can enter the pathway at different points. Gluconeogenesis has a variety of feeder molecules that it can use to make glucose. In terms of fats, only glycerol can enter gluconeogenesis, only glycerol. No fatty acids, steroids, anything like that. For amino acids, only lysine and leucine are unable to be gluconeogenic. They cannot be used for gluconeogenesis. However, all of the other amino acids are able to be used for gluconeogenesis. It's worth noting though that some of these are only able to contribute specific carbons. For example, the ketogenic amino acids are only able to contribute certain carbons to gluconeogenesis, and others are going to wind up as ketone bodies. Lastly, lactate, the molecule we were just talking about in the lesson, is the starting material for gluconeogenesis. And here you sort of have the overview of gluconeogenesis, and you can see that many of the same enzymes are being used. However, there are a few, 4 to be exact, new enzymes that we're going to talk about on the next page. But for now, let's just talk about the overview of what is required for gluconeogenesis. And it's basically very similar to glycolysis but opposite, of course. So, for glycolysis, you start off with 1 glucose molecule and 2 ATP, right? You need to burn 2 ATP in the preparatory or energy investment phase. And then from that, you yield 4 ATP. So that's going to be a net of 2 ATP, 2 NADH, and 2 pyruvate. And I should, for completeness' sake, say that you need 2 NAD⁺ to carry out glycolysis because, as we saw, NAD⁺ is a very important ingredient to glycolysis, as there's this whole fermentation process solely dedicated to regenerating NAD⁺ from NADH. So, this is glycolysis right here. And you can see that gluconeogenesis is almost the opposite, right? To make one molecule of glucose, you need to take 2 pyruvate, 4 ATP, 2 GTP, and 2 NADH. So you can see here that actually we're using more energy to build the glucose than we get from actually catabolizing glucose. So gluconeogenesis is more energy-intensive than glycolysis provides energy. But this is okay. This is okay. And, you know, for one thing, when we get to cellular respiration, you'll see that even though you might not be yielding all your ATP back from the glycolytic part, once you get over to oxidative phosphorylation, your ATP yield is just bonkers. So, this slightly extra amount of energy that our cells need to use to generate gluconeogenesis, they'll get a return on that investment from cellular respiration. However, gluconeogenesis is also sometimes used just to provide glucose for cells to continue glycolysis in periods of intense energy demand. Glycolysis and gluconeogenesis also both occur in the cytosol, which should make sense. That's where all the enzymes for glycolysis are. So of course, gluconeogenesis is also going to occur there. And a really important part to think about is that the two pathways are very tightly regulated. And this is because we don't want a futile cycle. So, let's think about this for a second. If we have looking at this figure, if we have glycolysis and gluconeogenesis running simultaneously, what's happening? Are we really generating anything? Every action taken by the glycolytic enzymes will be undone by the gluconeogenic enzymes. So, it's almost like we're spinning our wheels, right? We're running in place. We're expending energy, but we're not really getting anywhere for that. That's what we call a futile cycle, and our cells just can't afford to waste energy like that. So, these two processes are regulated. They're very tightly regulated, and they're regulated in such a way that when one pathway is running, the other is shut off. And this is also important from an enzymatic standpoint. If you think about the fact that many of these enzymes are being shared, well, these enzymes need some sort of oversight, telling them sort of which way they should be pushing those reactions. And again, for glycolysis, we have those 3 particular reactions that drive the whole pathway. So, the regulation of those particular enzymes is going to play a big role in actually controlling glycolysis and gluconeogenesis. You'll see that when those glycolytic enzymes are active, the specifically gluconeogenic enzymes will be inactive, and vice versa when the specifically gluconeogenic enzymes are active, those enzymes specific to glycolysis will become inactivated. And of course, again, those are the enzymes from reactions 1, 3, 10. All right, let's flip the page and actually talk about those reactions specific to gluconeogenesis.

Here’s what students ask on this topic:

What is gluconeogenesis and how does it differ from glycolysis?

Gluconeogenesis is the metabolic pathway that generates glucose from non-carbohydrate substrates such as glycerol, lactate, and most amino acids. It is primarily the reverse of glycolysis, which breaks down glucose into pyruvate. While many of the same enzymes are used in both pathways, gluconeogenesis requires new enzymes to bypass the three irreversible steps of glycolysis. These steps are catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase in glycolysis. Additionally, gluconeogenesis is more energy-intensive, requiring 4 ATP, 2 GTP, and 2 NADH to synthesize one glucose molecule from 2 pyruvate molecules. Both pathways occur in the cytosol and are tightly regulated to prevent futile cycles.

Which substrates can be used in gluconeogenesis?

Gluconeogenesis can utilize a variety of substrates to produce glucose. These include glycerol, which is derived from fats, and lactate, which is produced during anaerobic glycolysis. Most amino acids can also serve as substrates, except for lysine and leucine, which are exclusively ketogenic. Some amino acids contribute specific carbons to the gluconeogenic pathway, while others may end up as ketone bodies. This diversity in substrates allows the body to maintain glucose levels during periods of fasting or intense exercise.

Why is gluconeogenesis considered more energy-intensive than glycolysis?

Gluconeogenesis is considered more energy-intensive than glycolysis because it requires a higher input of energy to synthesize glucose. Specifically, to convert 2 pyruvate molecules into 1 glucose molecule, gluconeogenesis requires 4 ATP, 2 GTP, and 2 NADH. In contrast, glycolysis generates a net gain of 2 ATP and 2 NADH from the breakdown of 1 glucose molecule into 2 pyruvate molecules. This higher energy demand in gluconeogenesis is necessary to drive the endergonic reactions that reverse the irreversible steps of glycolysis.

How are glycolysis and gluconeogenesis regulated to prevent futile cycles?

Glycolysis and gluconeogenesis are tightly regulated to prevent futile cycles, where both pathways would run simultaneously, wasting energy without producing any net gain. This regulation is achieved through allosteric control and hormonal signals that ensure when one pathway is active, the other is inhibited. Key regulatory enzymes in glycolysis, such as hexokinase, phosphofructokinase, and pyruvate kinase, are inactivated when gluconeogenesis is active. Conversely, the enzymes specific to gluconeogenesis are inhibited when glycolysis is active. This reciprocal regulation ensures efficient energy use and metabolic balance.

Where do glycolysis and gluconeogenesis occur within the cell?

Both glycolysis and gluconeogenesis occur in the cytosol of the cell. This localization makes sense because many of the enzymes involved in these pathways are shared, and the cytosol provides the necessary environment for these metabolic processes. The shared location also facilitates the tight regulation required to prevent futile cycles, ensuring that when one pathway is active, the other is inhibited.